Pumps with a rotatable piston have the advantage that they can be constructed as control piston pumps, in which the piston acts as a rotary slide valve. Such pumps have a minimum of movable parts and are extremely robust. In order to assure the rotary slide valve function, the joint between crank and piston must be constructed such that angle changes are possible, however, the piston is rotated safely and as much as possible clearance-free. Thus, special demands on the joint are made.
In a known pump (German Pat. No. 509 222), the crank is connected to the piston through a ball-and-socket joint. The ball-and-socket joint is susceptible to wear because of the constant sliding movements between ball and ball socket, which can quickly result in clearance in the ball-and-socket joint. This results in noise development and also in changes of the conveying characteristic. Therefore, either a good lubrication must be provided or, however, a short life must be accepted.
Known are also pumps of the abovementioned type (EP OS No. 0 116 165), in which sliding movements are substantially avoided in the jointed connection between crank and piston, which results in a long life. One of the solutions described in the known reference has a conical surface rolling on a flat surface. It is true that one thereby obtains favorable pressure conditions between the rolling surfaces, however, only at a specific angle of inclination. Thus, the conveying stroke of said pump cannot be changed simply by changing the inclination of the axes. EP OS No. 0 116 165 also describes a solution (FIGS. 4,5 of this reference), which permits a change of the inclination. One rolling surface is thereby constructed as a torus the other one as a groove, into which the toric surface extends. A spring is provided for transmitting the forces during the suction stroke, which spring presses the toric rolling surface into the groove. The pressure conditions are not particularly favorable, since for structural reasons the cross section of the torus is small. Also it is not possible to transmit the forces occurring during the suction stroke through the rolling surfaces.
The basic purpose of the invention is to construct a piston pump of the abovementioned type such that the angle between the piston axis and the crank axis can be adjusted for the purpose of changing the conveying stroke and that at all adjusted positions favorable pressure conditions exist. The pump is also supposed to be able to be designed such that the forces occurring during the suction stroke can also be transmitted through the rolling surfaces.
This purpose is attained according to the invention by the rolling surfaces on crank and piston being constructed as troughs and by a convex rolling member being received in the troughs, the surface curvature of which rolling member is greater than the curvature of the trough surface, whereby the edges of the troughs are oriented such that during all relative positions between the troughs, the trough edges do not contact one another and a form-closed engagement between troughs and rolling member exists in peripheral direction of the rotating movement and forces can be transmitted through the rolling member in direction of the pressure stroke.
In contrast to the known pumps of the abovementioned type, the subject matter of the invention does not have rolling surfaces roll directly on the piston and rolling surfaces directly in the crank, but an additional loose rolling member is provided. The pressure relationships do not change in such a basic construction during adjustments of the angle between crank axis and piston axis, so that a change of the inclination can be provided. The pump is, however, also advantageous if an adjustment is not provided, since the rolling member and the troughs can be easily manufactured. Because of the difference in the curvature of rolling member and trough surfaces, the rolling member carries out a rolling movement simultaneously in both troughs, whereby sliding friction is avoided. The pressure conditions are advantageous in all adjustments. Since the curvatures of the trough surfaces and the curvature of the rolling member can come very close, the Hertz pressure is relatively small, which results in a long life of the joint even at poor lubrication.
The edges of the troughs are advantageously oriented such that a form-closed engagement between troughs and rolling member also exists in direction of the suction stroke, so that tension forces can also be transmitted from the crank onto the piston. Such a design has the advantage that a spring is not needed for overcoming the suction forces. The invention can, however, also be designed with an orientation of the troughs which does not permit the transmission of forces during the suction stroke. In this case it is necessary, just like in the known pump of the abovementioned type, to provide a spring which receives the forces occurring during the suction stroke.
The troughs are constructed preferably elongated and are oriented transversely to their paths of movement. The difference between the width of the circular crank path and the small axis of the ellipse, along which the trough which is provided on the piston moves, is bridged with such elongated troughs. An elongated construction of the troughs is not needed when the angle between crank axis and piston axis is small, since the difference between the diameter of the crank path and the length of the small axis of the ellipse is very small. An elongated construction of the troughs is also not needed in the yet to be explained tangential arrangement of the troughs.
The rolling member is preferably a ball. A ball-shaped rolling member can be particularly easily manufactured. Balls with good physical properties can be obtained in commerce. Well suited are balls, as they are also used in ball bearings. Shapes differing from a ball shape are possible. For example a disk-shaped rolling member can be considered.
According to a further development of the invention a preferably elastic clamp element is provided in order to hold the trough members containing the troughs against the rolling member. This prevents with certainty that the troughs lift off from the rolling member. However, a clamp element is not needed in every case. A clamp element, however, is needed when a form-closed engagement is not provided for the transmission of the suction stroke forces. In place of a clamp element, it is also possible to provide a special support between crank and piston, which is spaced from the piston axis and serves to transmit a negative torque, as it may possibly occur during the suction stroke.
If the angle between crank axis and piston axis is supposed to be changeable, a construction is particularly advantageous. Whereby by rotating a cylinder member containing the cylinder bore, the angle of inclination can be changed and be moved to the value zero, if the angles are chosen for example such that the angle between the axis of the cylinder bore and the axis of the cylinder member is of the same size as the angle between the axis of the cylinder member and the axis of the crank. A conveying does not take place any longer in this position.